Conventional optical fibers that transmit light are fabricated from quartz glass and plastics. Optical fibers produced from quartz glass have excellent light transmission property and are currently used for long-distance communications. Plastic optical fibers are inferior to quartz optical fibers in terms of light transmission property, but because of their high flexibility, light weight and good processability, their application in short-distance communication light guides or sensors is being studied.
Some applications of plastic optical fibers require high heat-resistance. For example, optical fibers used in an automobile optical data link system are required to withstand temperatures as high as 100.degree. to 120.degree. C. due to the heat from an engine compartment. However, conventional plastic optical fibers comprise a core of polystyrene or polymethyl methacrylate and, therefore, their maximum use temperature limit is as low as about 80.degree. C. At temperatures higher than 80.degree. C., these optical fibers comprising a polystyrene or polymethyl methacrylate core shrink and their light transmission property decreases, and at even higher temperatures, i.e., 100.degree. C. or more, these optical fibers further shrink and may break to make light transmission impossible.
In order to minimize such thermal shrinkage during use, plastic fibers are sometimes previously subjected to heat treatment. Though this heat treatment is effective in reducing the heat shrinkage of the fibers during use, the fibers themselves are no longer flexible and easily break due to vibration or bending, and thus lacking practicability.
It is considered that when plastic optical fibers are continuously used at temperatures as high as 80.degree. C. or more, they not only shrink by heat but also are susceptible to deterioration by oxidation. Oxidation of fibers lowers the light transmission property due to formation of a chemical bond that absorbs light. Therefore, the outer layer of plastic optical fiber is coated with a resin having a small percent oxygen transmission in order to prevent deterioration of fibers by oxidation as described in Japanese Patent Application (OPI) No. 162704/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and Japanese Utility Model Application (OPI) No. 170407/81. Resins having a low oxygen transmission disclosed therein include those resins which can be coated by melt-extrusion, such as polyamide, a saponification product of an ethylene-vinyl acetate copolymer (EVA), ethylene-vinyl alcohol copolymer, polyvinylidene chloride, polyvinylidene fluoride, etc. However, in order to coat such a thermoplastic resin by melt-extrusion, the extrusion must usually be conducted at temperatures as high as 200.degree. C. or more. Extrusion at high temperatures gives rise to problems such that the plastic optical fiber under the coating layer is extended by heat or the boundary surfaces of the fiber, core and cladding have non-uniform structures due to heating and cooling, resulting in increase of the transmission loss. These problems frequently arise particularly in obtaining a thick coating layer of the thermoplastic resin and, therefore, the extruded coating layer only has a thickness of about 50 .mu.m at most.
Further, the above-described plastic optical fibers coated with a thermoplastic resin have improved heat shrinkability at 80.degree. to 100.degree. C. as compared with non-coated plastic optical fibers, but they greatly shrink similarly to non-coated fibers at temperatures as high as 120.degree. C. or more, resulting in significant deterioration of flexibility and light transmission property.